Abstract:

We have investigated the saturated ethylene layer on Ni(1 10) by
low-energy electron diffraction (LEED), angle-resolved ultraviolet
photoemission spectroscopy (ARUPS), and near-edge x-ray-absorption
fine structure (NEXAFS). This layer exhibits a c (2 X 4) LEED pattern that
corresponds to a structure containing two adsorbates per primitive unit cell.
The ethylene molecules are adsorbed with the molecular plane parallel to
the surface and the C-C axis preferentially aligned along the [11OBAR]
direction of the substrate, as is independently determined from the ARUPS
and NEXAFS experiments. The two-dimensional (2D) adsorbate band
structure is determined from the ARUPS spectra at various photon
energies. Except for the pi-orbital, all ethylene-derived bands show
significant dispersion (up to 2 eV), but no splitting as would be expected for
a structure with two molecules per unit cell. The experimentally determined
band structure is reproduced in all details by extended-Huckel-theory
calculations for an unsupported ethylene layer. The structural model derived
from LEED, ARUPS, and NEXAFS is confirmed both by force field and by
the 2D band-structure calculations. This indicates that the
adsorbate-adsorbate interactions are essentially decoupled from the
adsorbate-substrate interaction, that is responsible for the chemisorption
bond.